Development of a high-voltage IC obtained by integrating a plurality of kinds of semiconductor devices on an SOI (Silicon on Insulator) substrate using the insulating/isolating technique is being advanced. A diode is mounted on such a high-voltage IC. A diode formed in such an SOI substrate is often constructed as a lateral diode. Each of patent literatures 1 and 2 discloses an example of the lateral diode.
A lateral diode has an n-type cathode region, a p-type anode region, and an n-type drift region partitioning between the cathode region and the anode region. The anode region has a first diffusion region of high concentration and a second diffusion region covering the high-concentration first diffusion region and having impurity concentration lower than that of the first diffusion region. The anode region is often constructed by two kinds of diffusion regions. Generally, the impurity concentration and diffusion depth of the first diffusion region of high concentration are set to make electric connection to an anode electrode excellent and adjust an injection amount of holes at the time of forward bias. The impurity concentration and diffusion depth of the second diffusion region covering the first diffusion region are set to adjust the injection amount of holes at the time of forward bias together with the first diffusion region and reduce concentration of holes flowing toward the first diffusion region at the time of reverse recovery.
For example, when the impurity concentration of the second diffusion region covering the first diffusion region of high concentration is high, although on resistance at the time of forward bias decreases, the amount of holes injected to the drift region increases, a reverse recovery charge amount increases, and switching loss increases. On the other hand, when the impurity concentration of the second diffusion region is low, although the amount of holes injected to the drift region decreases and switching loss decreases, the on resistance at the time of forward bias increases. Consequently, the impurity concentration of the second diffusion region is preferably set to a certain level of concentration. When the diffusion of the second diffusion region is deep, although the on resistance at the time of forward bias decreases, the amount of holes injected to the drift region increases, a reverse recovery charge amount increases, and switching loss increases. On the other hand, when the diffusion of the second diffusion region is shallow, although the amount of holes injected to the drift region decreases and switching loss decreases, the on resistance at the time of forward bias increases. Consequently, the diffusion depth of the second diffusion region is also preferably set to some degree of depth.    [Patent literature 1]: Japanese Unexamined Patent Application Publication No. H11-233795 (corresponding to U.S. Pat. No. 5,982,015)    [Patent literature 2]: Japanese Unexamined Patent Application Publication No. 2006-270034
As described above, in the anode region constructed by the two kinds of diffusion regions, the second diffusion region defining the contour of the anode region is constructed with the certain level of impurity concentration and the some degree of diffusion depth. Consequently, a corner part having a certain curvature is formed in the anode region. A high electric field is applied to the corner part in the anode region at the time of reverse recovery and holes in reverse recovery current flow. Due to this, at the time of reverse recovery, a dynamic avalanche phenomenon occurs around the corner part, and the carrier amount at the time of reverse recovery increases. As a result, the reverse recovery charge amount increases, and the switching loss increases.